Magnetic amplifier



April 10, 1951 R. w. OLSON 2,548,o49

MAGNETIC AMPLIFIER Filed July 10, 1948 Patented Apr. 10, 1951 MAGNETIC AMPLIFIER Robert W. Olson, Dallas, Tcx., assignor to Geophysical Service, Inc., Dallas, Tex., a Corporation of Delaware Application July 10, 1948, Serial No. 38,102

7 Claims. 1

This invention relates to a device for amplifying electrical currents and more particularly to a device that operates on magnetic principles to amplify currents of low frequency.

It is a well-known fact that an iron core inductance, which offers a certain amount of inductive reactance when subjected to a current of a particular frequency, will offer far less inductive reactance under the same conditions if the iron core is magnetically saturated by external means. t is also well-known that the application of relatively small amounts of direct current to a coil surrounding an iron core can be caused to effect such saturation. rI'his is particularly true where the alternating current is of relatively high frequency and the iron core can therefore be relatively small so that a Small direct current can effect saturation.

It is the purpose of this invention to apply this principle along with others to the construction of an amplifier for low frequency currents which are not readily susceptible to high fidelity amplification by other means.

It is also well-known that the current through a thermionic tube may be Controlled by controlling the temperature of the cathode of such a tube and it has been discovered that the relation of the current through the tube to the temperature of the cathode is such that as the temperature of the cathode increases the current through the tube increases slowly at first and then very rapidly and thereafter more slowly again, until the temperature of the cathode has been in- Creased to a point where destruction of some element of the tube takes place. It is a purpose of this invention to combine this principle with the magnetic principle set forth above for the construction of a magnetic amplifier of the type aboVe-mentioned.

The manner in which these principles are to be combined in .accordance with this invention to effect the desired results can best be explained and visualized by reference to a specifio Circuit and, therefore, a specific Circuit which will serve as an example of the application of the principles of this invention has been diagrammatically illustrated in the figure and will now be described.

In the Circuit of the figure the principal parts utilized are a core of magnetic material lt, for

current IZ; a plate supply 13 for the diode; and .z

the necessary windings and connections for lthe inductance core and other parts.

One lead from the source of alternating current [2 is connected by a conductor 2B to .one of the filament terminale of the diode I. The other of the leads from the source of alternating current 2 is connected through a pair of windings 2! and 22 on the magnetic core IG to the other filament terminal of the diode I. Thus the filament of the diode is heated by current from the source 12 and the amount of current supplied is Controlled by the inductive reactance of the coils 2! and 22 on the magnetic core lt. These coils and 122 are so wound and so connected that the inductiV-e reactance that they provide is added together rather than subtracted. A single coil can b-e used in place of the two coils shown in the figure but it is preferred to use two coils on a figure 8 shaped core anrl arrange these coils so that they introduce no appreciable flux into the cross-member of the core, and thus introduce no currents into coils wound `on that part of the core.

The alternating current supplied to the device may be of practically any frequency but preferably is of a fairly high frequency, for example a kilocycle or above, for this permits a reduction in the size of the core and hence makes the core more Sensitive to saturation by small direct currents.

The plate of the thermionic tube I is connected back vto one side of the filament of this vacuum tube through a. plate resistor 23, the plate supply battery E3 and a small coil 24 wound on the center leg of the inductance core W. Since only direct current can fiow through the thermionic tube i I, only direct current will flow in the small coil 24 on the inductance core ii] and this direct current Will tend to saturate the core..

If this direct current were great enough to saturate the core IO, the effect would be to reduce the inductive reactance of the coils 2| and `22 in the filament circuit, thus permitting a greater amount of current to flow through the filament, increasing the direct Current through the thermionic tube H and the direct current through the small coil 24, and thus starting a `Vicious cycle which would continue until the filament of the thermionic tube H either burned out or until the thermionic tube ii was Conducting as great a Current as was possible under the conditions present in the circuit.

To prevent this, a second small coil e is wound on the center leg of the magnetic core IB and is supplied with direct current from a battery 26 through a variable resistance 21 which serves for calibration and adjustment of the device. It is not changed during the making of measurements. This second small coil is so wound and so supplied with current that the magnetic flux which it induces in the inductance core if! is opposite of that induced by the first-mentioned small coil 24 and the magnitude of this current is adjusted to be such that prior to the flow of any current in the thermionic tube l the inductance core ll] will be at least partially saturated by the effect of this second small coil 25 and the current supplied to it. Thus, as the thermionic tube Il begins to conduct current, and current therefor passes through the first-mentioned small coil 25:, the effect of the small coil 2ii will be to buck the effect of the second small coil 25 and lower the saturation of the core li] to a partially saturated state, thus raising the inductive reactance of the coils 2| and 22 in the iilament circuit and, hence, lowering the current to the thermionic tube ll until a balance is reached at which time current flow in the circuit will be stabilized with the core in a partially saturated condition.

Now in order to use this circuit as a magnetic amplifier, a third small coil 28 is wound on the center leg of the inductance core IE? and is connected to a pair of input terminals il and 3l.

When a current is passed through the third small coil 28, that is, When the current is introduced into the terminals 30 and 3l, the circuit will still immediately come to equilibrium but it will be necessary at equilibrium for the thermionic tube Il to pass either a greater or less amount of current to offset the magnetic effect produced by the third small coil 28 and the current to be measured which is passing therethrough.

Now if the number of turns in the first small coil 2d, which is in a plate circuit of the thermionic tube, is less than the number of turns in the third small coil 23, which is in the input circuit, it Will be necessary for the current 'through that coil to be changed to a greater extent than is the current through the input circuit coil 28, in order to return to the point of equilibrium. Thus, as the input current is changed, the current in the plate circuit of the thermionic tube will change to a greater eXtent. This will effect a change in the Voltage drop across the plate resistor 23 and, by connecting output leads 32 and 33 across this plate resistor 23, it is possible to obtain an amplified response from any changes made in the input to the device through the terminals 3B and 3|. The reading in this case is, of course, the voltage drop across the resistor and is proportional to the current flow therethrough. This voltage may be quite high in comparison to the voltage causing the current under observation to flow.

-Exactly the opposite effect, that of deamplification, may be obtained, if desired, by making the small coil 24 in the plate circuit of the thermionic tube have a greater number of turns than the small coil 28 in the input circuit. This effect also reduces the impedance of the output circuit as compared to the input circuit of the device which is often highly desirable.

Such a circuit as that described above has been found to respond with great accuracy and Without appreciable drifting to very low frequency currents and to be capable of yielding quite high amplification ratios. Preferably, the amplifier is adjusted to operate on the steepest 4 part of the filament voltage-plate current characteristic curve of the thermionic tube and when so Operating is extremely Sensitive.

An ammeter 34 has been placed in the circuit of the thermionic tube and by using this animeter as a guide the Variable resistance 21 may be adjusted to vary the potential applied by the battery 26 to the biasing coil 25 so as to make the thermionic tube operate at the desired point on its characteristic curve. The voltage or current to be amplified may then be applied to the terminals 30 and 3| and the amplified output will appear across the output conductors 32 and 33. By eliminating the plate resistor 23, and connecting the plate of the tube ll directly to the positive terminal of the battery |3, the amplied current may be read directly from the meter 34.

Many mnor modifications may be made within the scope of this invention; for example, other tubes than diodes may be utilized, indirectly heated cathodes may be utilized where the frequencies to be ampli'ed are extremeiy low and the various coils may all be wound on a single straight magnetic core. Other possible modifications will immediately be apparent to those skilled in the art.

The current sensitivity of the device of this invention increases with an increase in the frequency of the alternating current that is supplied for its operation, since as previously explained an increase in the frequency of this alternating' current permits a reduction in size of the inductance core and thus makes possible an inductance core that is more Sensitive to saturation by direct current. By using a relatively high driving frequency, say of the order of a hundred kilocycles or more, the device may be made so Sensitive to changes in direct current that it can be used practically with thermocouples which have very low voltage outputs and produce very small currents. In such instances, it is probably desirable to use a higher resistance in the plate circuit in order to get a high voltage proportional to the Very small voltage generated by the thermo-couple itself.

What is claimed is:

1. A device of the type described that comprises a source of alternating current, a saturable core inductance and a thermionic tube, the heating element of the thermionic tube, the inductance and the source of alternating current being connected in series; a conductor coil positioned so as to induce magnetic flux in said saturable core, a source of direct current potential and a current indicator connected in series therewith between the plate of said thermionic tube and the cathode of said tube; a second conductor coil also positioned so as to produce flux in said saturable core, and means to supply a constant current to this coil; and a third conductor coil also positioned so as to induce fluX in said saturable core and connections for passing a current to be amplified through this third coil.

2. A device of the type described that comprises a figure 8 shaped saturable magnetic core, a pair of inductances positioned on the opposite ends of said core, said inductances being electrically matched and connected in series aiding relationship, a source of alternating current, a thermionic tube, and conductors connecting the source of alternating current, the primary coils land the heating filamen-t of the thermionic tube in series; a conductor coil on the middle leg of said core, a connection from one end of said coil to the cathode of said thermionic tube, a source of direct current, an electrical indicating device, and connections for connecting the other end of said coil in series with said source of direct current and said signal-indicating device to the plate of said thermionic tube; a second conductor coil also on the middle leg of said core, a source of direct current, and means for adjusting the amount of said current, all so connected as to supply a constant but adjustable amount of direct current to said second coil; and a third conductor coil also positioned on the middle leg of said core and connections to said third coil for introducing therein a current to be amplified.

3. A device as defined in claim 1 further characterized in that a resistor is included in the plate circuit of the thermionic tube and connections are provided at opposite ends of said resistor for receiving the voltage developed across said resistor.

4. A device as defined in claim 2 further characterized in that a resistor is included in the plate circuit of the thermiono tube and connections are provided at opposite ends of said resistor for receiving the voltage developed across said resistor.

5. A device as defined in claim 1 further characterized in that the third coil contains a larger number of turns than the first coil thus causing a greater current to change in the plate circuit of the thermionic tube than occurs in the third coil.

6. A device as defined in claim 2 further characterized in that the third coil contains a larger number of turns than'the first coil thus causing a greater current to change in the plate circuit of the thermionic tube than ocours in the third coil.

7. A device as described in claim 1 further characterized in that the third coil has a lesser number of turns than the first coil thus causing the plate current of the thermionic tube to vary to a lesser degree than the signal current.

ROBERT W. OLSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,808,522 Brown June 2, 1931 2,236,195 McKesson Mar. 25, 1941 

